1. Field of the Invention
This invention relates to laser diode imagesetters having multiple resolution modes, and more particularly, to a method and apparatus for stabilization of the optical output power of a laser diode during the trace portion of a scan across a photosensitive material, irrespective of the resolution mode selected for the imagesetter.
2. Description of the Prior Art
Conventional laser imagesetters (e.g., typesetters or printers) have generally used lasers, such as helium-neon lasers, which generate a light beam used for recording or reproducing an image or text, either directly e.g., by exposing a photosensitive film or paper or indirectly, e.g., by photosensitive charging of an electrostatic recording device. Not only are such lasers and their accompanying power supply, modulator and modulator driver relatively expensive and power hungry, but their peak modulation rate is limited by the size of their output light beam. Thus, additional optical components are necessary, involving additional costs, in order to provide an improved modulation performance required for a high resolution imagesetter.
In contrast thereto, currently available semiconductor laser diodes in the milliwatt range can be readily modulated at rates up to 20 MHz by modulating its driving current. Consequently, it is desirable to use a semiconductor laser diode as the source of a modulated light beam for recording in a high resolution imagesetter. However, the use of a semiconductor laser diode in an imagesetting apparatus does have a drawback. That is, the optical power output of a laser diode is sensitive to changes in temperature of the laser diode. Thus, unwanted changes in the optical power output of the laser diode due to changes in temperature, can easily result in changes of the amount light energy which reaches the photosensitive material and thereby produce unwanted image variations. Although one would think that placing the laser diode in an oven or other temperature stabilized environment would solve this problem, it does not, since the temperature of the laser diode also changes due to heating effects inside the semiconductor structure of the laser diode.
In the past, relatively slight changes in optical power output due to internal heating of the laser diode was not a problem, however, with new high resolution imagesetter g. of the type described in U.S. Pat. No. 4,719.474, filed July 19, 1985 in the names of Hansen and Klepper, even slight variations of the optical power output can result in undesirable image distortions. This high resolution imagesetter is designed for a standard resolution of 1,200 dots per inch (dpi) when using a infrared photosensitive material such as the KODAK PAGI-SET IR (a trademark of the Kodak Company) phototypesetting paper available from the Eastman Kodak Co., Rochester, NY. This paper requires that the laser optical power output be such that 50 ergs/cm.sup.2 reach the paper in order to make a spot thereon. The combination of the scanning operation of the light beam and the sensitivity of the photosensitive material is such that at the standard resolution, a spot of 22 micrometers is formed. The solid line curves in FIG. 1 illustrate vertically adjacent spots and show that the spot shape is generally Gaussin. The scanning operation is adjusted so that adjacent scan lines generally overlap at about their half-intensity (50%) points and results in a spot energy of 50 ergs/cm exposing the photosensitive material.
When the optical power reaching the photosensitive material is increased, as illustrated by the dashed line curves in FIG. 1, the point of overlap with a vertically adjacent spot, which is still at the 50% power level due to the scanning system, corresponds however to a greater optical power output and hence a larger spot size. This affect seems intuitively obvious, since the greater the power of the light reaching the photosensitive paper the greater the exposure and thus, the larger the diameter of the spot formed thereon. This change in spot size will cause an undesirable change in the exposed image size and/or density depending on the composition of the exposed image. It has been experimentally determined that laser optical power changes greater than even one percent can cause undesirable image distortions.
Conventional techniques for stabilizing the optical power output of a laser diode use a sample and hold circuit which samples the laser optical power output during each scan retrace interval and then provides an amount of drive current to the laser during the next scan trace interval which results in a laser optical power output corresponding with the sampled optical power output during the previous retrace interval. Although such stabilization techniques were sufficient in the past, the high sensitivity of the present day photosensitive material and high resolution requirements of todays high quality imagesetters, requires a greater degree of optical power output stabilization.
Laser diode optical power output fluctuations can result from changes in the internal operating temperature of a laser diode, which changes are a function of its applied drive current. Since its drive current is determined by the image-representative video signal which is applied to modulate the laser diode, the rate at which the laser diode temperature changes is less than a scan trace time. Therefore, these power fluctuations cannot be compensated for with the previously noted sample and hold technique. The sample and hold technique can only compensate for slower changes in temperature. Consequently, modulating the laser diode with a constant current during the scan trace (determined by the sample and hold circuit during the scan retrace interval) is not sufficient to stabilize the laser optical power output to the degree required for a high quality imagesetter.
The variation of image density can easily be noticed in certain types of image compositions. A worst case condition is representatively illustrated in FIG. 2, where a vertically oriented fine tint screen on the far right (near the end of scan trace) is alternately preceeded by a blank area (horizontal white bar) and then by a fully exposed area (horizontal black bar). The laser optical power output during the tint portion of the image will be slightly different between successive scan lines in which the laser has been OFF and in which the laser had been ON during the preceeding portion of that line scan. As previously noted, if the optical power output of the laser diode changes, the effective spot size on the photosensitive material will also change. Consequently, with an image composition such as illustrated in FIG. 2, a density variation is created in the tint portion of the image which appears as a horizontal "banding" within the vertical tinted portion.
It is desirable to provide a method and apparatus for stabilizing the optical Power output of a laser diode in a multi-resolution imagesetter during the scan trace portions of its operation, which method and apparatus will also take into account changes in laser beam optical power required for different resolution modes of the imagesetter.